To cope with increase of communication traffic, increase of speed and capacity of an optical transceiver is demanded. In recent years, the optical transceiver increasingly introduced uses a digital coherent technology that is combination of digital signal processing (DSP) and coherent detection.
The optical transceiver of a transmission rate of 100 Gb/s per one channel uses, for example, a modulation system of PDM-QPSK (polarization division multiplexing-quadrature phase shift keying) at Baud rate of 32 Gbaud. An optical transmitter modulates each of linear polarized light (X-polarized wave and Y-polarized wave) orthogonal to each other by QPSK baseband signals to generate PDM-QPSK optical signals. An optical receiver converts the received optical signals into baseband signals through the coherent detection of the optical signals and local oscillation light, and demodulates the QPSK by the digital signal processing (DSP) to reproduce transmission data.
To increase the transmission capacity per one channel, the optical transceiver of a transmission rate of 400 Gb/s uses, for example, a modulation system of PDM-16QAM (polarization division multiplexing-16 quadrature amplitude modulation) at Baud rate of 64 Gbaud, or a modulation system of PDM-64QAM at Baud rate of 43 Gbaud. As described above, in the future optical transceiver, increase of Baud rate and multi-valuing of the modulation system progress in order to increase the transmission capacity per one channel.
The optical transceiver is required to have excellent transmission characteristics in a wide range along with increase of Baud rate and multi-valuing. The transmission characteristics of a transmission signal in the optical transceiver are represented by a transfer function. The optical transceiver generally includes a plurality of lanes (in-phase component XI of X-polarized wave, quadrature component XQ of X-polarized wave, in-phase component YI of Y-polarized wave, and quadrature component YQ of Y-polarized wave), and is required to sufficiently suppress difference of the transfer function among the lanes because the difference of the transfer function among the lanes causes deterioration of total transmission characteristics of the system. In a case where frequency characteristics of the transfer function of the optical transceiver are insufficient or a case where the frequency characteristics are different among the lanes, it is necessary to compensate the transmission characteristics or the difference among the lanes by, for example, the DSP. On this issue, a method in which chromatic dispersion of an optical transmission line or the difference among the lanes on reception side is compensated by the reception side (for example, see NPLs 1 and 2), and a method in which the difference among the lanes on transmission side is compensated by the transmission side (for example, see PTL 1 and NPL 3) have been proposed.